Automated Large-Scale Serial Inspection

Snecma Propulsion Solide designs, produces and markets rocket engines and composite materials for the defense, aviation and aerospace industries and other related fields.

Within the French national defense programs M45 and M51 as well as the European space programs such as Ariane 5 and Vega, Snecma is a first-choice business partner.

Snecma is a market leader in the manufacturing of jets and propulsion systems for strategic and tactical missiles as well as carrier rockets and fighter planes. In addition, high performance composites for rocket, plane and military propulsion systems, equipment and armament for manufacturing plants (thermal processing, semi-conductors, glassworks or chemical) or sensors for aerospace and military rockets, satellites, dynamometers or nuclear power plants are also produced.

With the armament and aerospace industries, manufacturing precision and the assembly of parts are hugely important. Damien Darriet is in charge of service methods and non-destructive dimensional control at Snecma. His department tackles the development of the solutions for the dimensional and material inspection, while at the same time focusing on modifying the existing equipment in order to keep the processes as simple as possible for the equipment users. Darriet explains: “How we perform dimensional inspection at Snecma is often determined by very unusual factors. The pyrotechnical regulations are extremely tight. For example, no more than 5 people are allowed to work on a single piece of equipment at any one time. The accuracy requirements are just as stringent. The internal rules mandate that the measurement uncertainty of the measurement equipment (standard deviation) is 16 times better than the assembly accuracy. Besides, the sheer size of our equipment – up to 12 m in height and 50 tons of weight – limit the kind of measurement equipment we are able to use.”First priority: Cutting measurement cycle times

As Snecma started this strategic activity, only theodolites were capable of meeting the requirements resulting from the size of the equipment that was being measured. The first theodolites installed were made by Kern, and were mounted onto the constructions with which the coordinates of points found on the equipment could be assigned to spatial segments in the room. This way, the metrology technicians inspected the jets found on the Ariane 5 rocket. After Kern was acquired by Leica Geosystems, the theodolites from the TM5100A series were deployed, and were used to inspect the different stages of the M51 rocket. Darriet expands: “The Leica TM5100A was exactly what we needed to inspect spatial sections with large dimensions – but the preparations and the actual measurements took too long. We had to take aim at the large measurement objects and then deploy three theodolites and a PC. The theodolites were located on heavy-duty industrial tripods at a height of one to three meters. We needed to use a hoisting platform for our personnel. At least four people were needed to inspect a single measurement object. Simply put, it took over a week to measure a single stage of the M51 rocket.”

During a theodolite training session – offered regularly by Leica Geosystems – Snecma started looking for a more automated measurement solution that would substantially reduce inspection times while staying within the accuracy requirements. A solution relying on a laser tracker seemed to be the most appropriate, but three issues had to be cleared beforehand: the Snecma team knew little about the laser technology, the effects of laser on a pyrotechnically charged object were unknown, and the size of the measurement object did not allow for performing all measurements without changing the location of the laser tracker. The automation was a massive challenge. Darriet explains: “Once a program has been defined, being able to run it over and over again means never having to worry about it again. Minimizing operator involvement during the chain of events in a measurement means maximizing the stability of the measurement results.”

The four companies that answered the bid invitation had to offer a long-term solution that would run for at least 15 years and was easy to use. Darriet remembers the solution proposals submitted by two other laser tracker manufacturers and a company that offered a solution based on photogrammetry: “The first solution required the acquisition of an articulated arm and a tracking laser. After several trials, it became apparent that this solution lacked the required accuracy. Besides, we would have to bring the arm very near to the measurement object, which would have been very difficult to implement in our sensitive surroundings. Hence, the first system was out of the running. The second company presented a system based on photogrammetry that stipulated using several networked systems and precise homing in onto the measurement objects. The inspection times were well beyond what was acceptable, and such a system didn’t let us inspect points from above. The third solution was another laser tracker, whose large acceptance angle gave it an important advantage of being able to inspect the measurement object from both the bottom and the top. And even though the distance meter in this particular laser tracker required fewer manual interventions than the previous model by the same manufacturer, the product as a whole was just not technically mature enough. The fact that no camera was offered with the system brought it a major disadvantage, further influencing our decision to steer away from that product.”

The laser tracker from Leica Geosystems features a video camera that allows the visualization of the entire measurement process, thus helping with the automation. For example, the operator can directly steer the laser beam to those reflectors that the system cannot find on its own – the operator has the same point of view as the laser beam. Using the theoretical or the approximate data about the points being inspected, the system searches for and measures the points indicated by the reflectors, regardless of their distance. During the measurements, only the laser tracker and the measurement object are located in the room. The entire control system, comprising a PC and a controller, are located in a different room. That way, the object being measured is not visible from the outside. Because of that, the installed camera provides a big advantage for Snecma and is indispensable during measurements.

A completely integrated measurement system

The installation of the laser tracker was performed by the Leica Geosystems commercial and service support staff in close cooperation with Snecma. The result is at once both sophisticated and astoundingly simple. Leica Geosystems transferred the implementation of the measurement software and the personal project supervision to a partner software firm. The laser tracker is installed on a lifting stand while the measurement object is mounted on a turning platform, both of which further automate the measurements. Both the lifting/lowering of the stand as well as the platform-turning functions are automated.

Software developers integrated additional functionality into the existing Axyz software platform, allowing for complete system automation. Among others, the user can set measurement point definition, the position of the laser tracker on the lifting stand and the measurement object angle on the turning platform. Prior to contracting this task out to the software partner, the calculations of the three different height positions of the laser tracker on its lifting stand were simulated in order to guarantee the correct positioning and to qualify the measurement uncertainties at every point. Then, the procedure was repeated with the actual elements at each measurement side. Numerous tests with a triple control of the measured points on objects reaching up to 11 meters in size yielded excellent results.

As part of this projects, special precision tools were developed to ease the measurements of hard-to-reach points and elements – under direct Snecma supervision and by deploying 220 TBR Leica Geosystems reflectors in order to materialize all measurement points on the object. Additionally, 25 cat-eye reflectors and two reflectors propped up on tools constitute the complete measurement network. A reference framework of the characteristic points on the measurement object itself is also integrated. All measurements are performed within this reference framework and can be followed in real time on a computer screen. “Inspecting our largest measurement objects never takes more than a day. With the integrated system solution by Leica Geosystems, we were able to maximally automate the application, thus fully accomplishing our stated goals. The operators are extremely satisfied with both the measurement cycle and its dependability,” says Darriet.

Running on autopilot

Two different user groups are involved with the laser tracker system at Snecma: those who install the tools and those who use the measurement system. Starting with the initial discussions and taking into consideration the actual application and the final requirements’ set for serial measurements, tools were developed that made a custommade solution possible.

“System adaptation was done according to the needs of the operators. They immediately showed great interest and were fully motivated. A laser tracker that can measure with accuracies of one hundredth of a millimeter, coupled to visually presenting the coordinate points on a computer screen when the reflector is moved from point to point, mean true technical progress for the operators. That’s the reason why the users were immediately highly motivated to take part in developing the system from the perspective of the machine-human interface. Thanks to their substantial contributions, the initial operation of the system was simple to implement, and the measurement system gained immediate acceptance,” Darriet explains and continues: “Even though the measurement system appears to be simple, the personnel using it to inspect measurement objects still have to have a full understanding of the technology behind it. Certain inspection jobs require 14 stations – meaning they require 14 observation points – all of them at different heights. Setting up the program, even when everything is simplified as much as possible, still requires a solid grasp of geometry and mathematics in order to produce good measurement results. One weeklong training session about the laser tracker use and its geometric principles guiding its operation were enough to understand how it functions. Our user group’s top priority was to learn the exact measurement procedure and to have an easy-to-use measurement instrument. They are extremely satisfied.”

The laser tracker system fully automatically calculates all parameters and creates measurement reports in Excel in order to visualize the individual points and to facilitate easy data manipulation with simple mathematical methods. The software is continually updated. The close cooperation, which was truly to the users’ satisfaction, has been very fruitful. “Nowadays, programming the machine-human interface is so simple that only the information about the measurement platform, where the positioning stand should be located and how it is positioned at the opposite side, is needed. You can see these points and their coordinates at a station. The automatic visualization of all points and the various measurement stages is easy to see on a single interface – a single computer screen. More detailed information is easily obtained,” points out Darriet.

Snecma currently uses two laser tracker systems from Leica Geosystems for its serial inspection requirements. To assure the ability to react promptly to different situations, the company has signed an on-site calibration contract. Also included in the contract are the setup and the adjustments to the laser tracker controller units. “It’s a complete package, including maintenance and calibration,” states Stéphane Malet, Hexagon Manufacturing Intelligence France sales engineer, who takes care of the Leica Geosystems customers in southeastern France. The systems have been in operation for the past five years, without the slightest cause for concern.

Multifarious measurement system

Using a laser tracker lets Snecma conduct accurate and dependable measurements of aerospace parts. Darriet explains: “According to their definition, aerospace parts require a direct inspection relative to a digital model. Measuring individual points and fine-adjusting the object surface allow for immediate comparisons to digital models. During the manufacturing process, newly assembled parts have to be thermally treated in a special oven, whereby the temperature plays a pivotal role. The temperature and the pressure change the geometric form of an object. After being put through the oven, a part needs to be inspected again. With certain sheeting parts, as little as 500 g of pressure are enough to deform it. The dimensional inspection has to be conducted briefly before the part enters the oven, and then shortly thereafter. Only a contactless measurement system located next to the part can solve this challenge. The Leica Geosystems laser tracker, matched to the hand-held, large-volume Leica T-Scan scanner, was the perfect solution. The first measurement objects were inspected through Leica Geosystems directly, and additional parts were entrusted to a service company. It quickly became apparent that the extra costs resulting from having to re-manufacture those parts that were not inspected and do not correspond to the CAD data are several times higher than an investment in an entire measurement system! Snecma is considering ordering a Leica T-Scan system.

Damien Darriet sums up: “In our business, contactless, high-accuracy inspection is the future of metrology. Not only the departments engaged in product development are aware of this, but also the colleagues who research the behavior of different materials in various production cycles under high temperatures and pressures or inspect them for material deposits or stability. Today, Leica Geosystems is the only manufacturer that can deliver metrology equipment that corresponds to our requirements in both the accuracy and the measurement volume.”